Tutorial Exactly Center an Image/Div Horizontally and Vertically
.center {
width: 300px;
height: 300px;
position: absolute;
left: 50%;
top: 50%;
margin-left: -150px;
margin-top: -150px;
}
Negative margins are exactly half the height and width, which pull the element back into perfect center. Only works with elements of a fixed height/width.